1 Tor Harold Percival Bergeron David M. Schultz Cooperative

Tor Harold Percival Bergeron
David M. Schultz
Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma,
and NOAA/National Severe Storms Laboratory, Norman, Oklahoma, USA
Robert Marc Friedman
Department of History, University of Oslo, Oslo, Norway
An entry in the New Dictionary of Scientific Biography
Editor-In-Chief: Noretta Koertge
To be published in 2007 by Charles Scribner’s Sons
19 December 2005
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Bergeron, Tor Harold Percival (b. Godstone, Surrey, England, 15 August 1891; d.
Uppsala, Sweden, 13 June 1977), synoptic meteorology, cloud and precipitation physics,
weather forecasting.
Bergeron was one of the principal scientists in the Bergen School of Meteorology, which
transformed this science by introducing a new conceptual foundation for understanding
and predicting weather. While developing innovative methods of forecasting, the Bergen
scientists established the notion of weather fronts and elaborated a new model of
extratropical cyclones that accounted for their birth, growth, and decay. Bergeron is
credited with discovering the occlusion process, which marks the final stage in the life
cycle of an extratropical cyclone. Bergeron also contributed to cloud physics, most
notably the description of the Bergeron–Findeisen process by which precipitation forms
inside a cloud containing both ice crystals and water droplets.
The Early Years
Bergeron was born in England to Swedish parents and raised in Sweden. His mother
knew Nils Ekholm, director of the Swedish Meteorological Institute (SMI), which proved
valuable for the young Bergeron. After receiving his B.Sc. from the University of
Stockholm in 1916, Bergeron spent the summers taking observations of visibility at
different locations around Sweden and returning to SMI in Stockholm during the autumn
to complete his research. He found that changes in visibility seemed to be related to
wind-shift lines (what would be later called fronts). On 1 January 1919, Bergeron
received the title of “extra assistant meteorologist” at the reorganized SMI, now called
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the Swedish Meteorological and Hydrological Institute (SMHI). Within a few months,
the tiny core of the incipient Bergen School, father and son Vilhelm and Jacob Bjerknes
and Halvor Solberg, recruited Bergeron to Bergen, Norway to join a new weather
forecasting service.
The Bergen School and the Occlusion Process
In 1917, the Bergen Museum (precursor for Bergen University) called Vilhelm Bjerknes
to a new professorship in meteorology. Bjerknes had been working in Leipzig on a
research program for creating an exact physics of the atmosphere and ocean. In contrast,
meteorologists at the time predicted weather primarily by often inaccurate empirical rules
of thumb and statistical insight. Upon coming to Bergen in 1918, Bjerknes organized an
experimental weather prediction service, directing a number of enthusiastic young
scientists in developing new forecasting practices based on insight into physical
processes. The impact of the work performed in Bergen, combined with the incubation of
several high-quality scientists, had an immense impact internationally on the burgeoning
scientific field of meteorology.
Much of the earliest work in Bergen focused on understanding the structure of
extratropical cyclones. Based on the first summer’s forecasting, Jacob proposed in
November 1918 a new model for these disturbances which accounted for their
asymmetric distribution of precipitation (Fig. 1). The basic structure was described as a
counterclockwise swirl of air around the low-pressure center. Warm air advancing from
the south rose up over cold air retreating northward on the east side of the low center.
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The boundary between the two was ultimately called a warm front. On the southwest
side of the low center, dense cold air advancing from the north lifted the warm air,
forming a boundary later called a cold front. The recently ended World War I inspired
using the word front to describe battle lines of advancing and retreating air masses.
[Bergeron would later suggest the symbols now used for cold and warm fronts (lines with
filled triangles and semicircles, respectively) on a postcard to Jacob Bjerknes on 8
January 1924.]
During the fall of 1919, Bergeron noticed that the cold front at times seemed to catch up
to and overtake the warm front, what he dubbed sammenklapping (roughly “coming
together” or “closing up”). He intuited that the cold front probably rode aloft over the
warm front, but he remained puzzled over the nature and significance of this finding. It
was not clear whether sammenklapping entailed an evolutionary component of
extratropical cyclones or simply a local geographical effect. Furthermore, Jacob resisted
changes to his model.
While in Stockholm and Bergen, Bergeron returned on occasion to this baffling
phenomenon. International efforts to increase the amount and frequency of weather data
enabled Bergeron to bring into clearer focus the cyclone’s structure. He arrived at a
convincing three-dimensional representation by which a cold front and warm front
merged, resulting in the previously sandwiched warm air being lifted aloft. Without
access to the warm air fueling the storm, such a cyclone would weaken. Eventually,
Bergeron used the term occlusion for this process, and the resulting boundary between
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the two cold air masses was called an occluded front. By 1922, he convinced Jacob of the
importance of this process to the evolution of extratropical cyclones. This discovery,
along with Solberg’s concept of cyclone families, changed the Bergen cyclone model
from a static conceptualization (Fig. 1) into one that featured the entire life cycle of birth,
maturity, and death (Fig. 2). Forecasters and theoreticians now had a model to help them
understand the processes affecting storm intensification and decay.
Occlusion is a seminal feature of the classic 1922 paper by Jacob and Solberg, “Life
Cycle of Cyclones and the Polar Front Theory of Atmospheric Circulation,” yet Bergeron
was not a co-author. At the time, Bergeron was in Stockholm where he was preoccupied
with other tasks, including preparation of a supplemental manuscript, which never was
completed. Bergeron was a perfectionist, oftentimes not completing publications for want
of further analysis. And whereas Solberg and the Bjerkneses accepted the need to
simplify when presenting the new findings, Bergeron aimed to depict all the new insights
in their full complexity. By temperament and principle, he could not easily collaborate
with the others in writing what he considered a much too hastily prepared publication.
Although he was not a co-author, his Bergen colleagues always gave him full credit in
discovering occlusion, the capstone of the Bergen school’s early achievements.
Indirect Aerology and Air-mass Analysis
To convince others of the reality and importance of fronts, the Bergen meteorologists
needed to create systematic methods to reproduce them reliably in daily forecasting work.
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Although all members of the emerging Bergen school contributed towards this goal,
Bergeron played a crucial role in establishing innovative forecasting practices.
Bergeron collaborated with Swede and close friend Ernst Calwagen to bring greater
clarity to an ever-growing number of new but elusive phenomena emerging from the
analysis of weather maps. To achieve this goal, they sought to refine and systematize the
Bergen group’s innovative methods for observing and analyzing weather. They brought
to maturity a method of indirect aerology, which allowed identifying fronts and tracking
the life history of large homogeneous bodies of air called air masses. At a time when
direct measurement of the atmosphere through weather balloons and kites above the
surface (aerology) was not available for daily forecasting, this method combined
observation of the clouds and sky overhead with analyses of phenomena plotted on the
weather map to envision the physical processes occurring in a three-dimensional
atmosphere. With the help of the Norwegian military air forces, Calwagen began to
supplement the indirect aerological methods with direct vertical measurements in the
atmosphere. While taking observations on 10 August 1925, Calwagen was killed after the
airplane fell apart in mid-air. Calwagen’s death deeply affected Bergeron—he never flew
again. Bergeron took over his friend’s work, integrated it with his own, and helped bring
the methods of air-mass analysis to maturity. Indirect aerology enabled the Bergen
meteorologists to achieve more accurate and detailed predictions, as well as to gain
insight into the nature of fronts, cyclones, and air masses. For Bergeron as well as other
members of the Bergen school, so-called synoptic meteorology was a legitimate means
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for winning new knowledge of the atmosphere, an equal partner to theoretical and
mathematical study.
The Bergeron–Findeisen Process
When Bergeron returned to Bergen in 1922, he stopped for several weeks at a health
resort at Voksenkollen, a hill north of Oslo often enclosed by fog. Bergeron noted that
when the temperature was well below freezing the roads through the forest were clear of
fog (Fig. 3). When the temperature was above freezing, however, the fog would extend
down to the ground (Fig. 3). Bergeron recognized that the saturation water-vapor
pressure over water is higher than that over ice at temperatures below freezing. Thus, he
surmised that diffusion of water vapor from evaporating supercooled liquid-water
droplets in the fog to frost growing on the trees might have been occurring to disperse the
fog. Although Alfred Wegener had already argued in 1911 that such growth was possible
in a cloud with both ice and water droplets, Bergeron was the first to recognize that this
could lead to precipitation. These ideas would be briefly developed in his doctoral thesis
in 1928, and presented more fully in 1933. Coupled with experimental confirmation by
the German Walter Findeisen in 1938, this process of forming precipitation in a cloud
possessing both ice crystals and supercooled liquid water droplets was eventually called
the Bergeron–Findeisen process (sometimes called the Wegener–Bergeron–Findeisen
process). This discovery promoted the subsequent growth of cloud physics as a vital
subdiscipline, not the least by providing a means to dissipate fog and a physical
mechanism for precipitation enhancement through cloud seeding.
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Apostle of the Bergen School
More than any other member of the Bergen School, Bergeron provided the detailed case
studies, lectures, and travel needed to develop grassroots support abroad for the Bergen
School concepts and methods. His role as apostle was facilitated by his linguistic talents:
he spoke seven languages and knew some of three others.
While employed during the early and mid 1920s by the Norwegian Meteorological
Institute, Bergeron spent time in Leipzig, working with Gustav Swoboda to demonstrate
the applicability of the Bergen School concepts in an analysis of a weather event over
Europe. “Wellen und Wirbel an einer quasistationären Grenzfläche über Europa” [Waves
and Vortices at a Quasistationary Frontal Surface over Europe] (1924) was the first
detailed publication to use the methods developed in Bergen. Bergeron wrote Part I of
his “Über die dreidimensional verknüpfende Wetteranalyse” [ThreeDimensionally Combining Synoptic Analysis] in 1928, for which he received a doctoral
degree from the University of Oslo. That paper covered many disparate topics, including
air-mass analysis and frontogenesis (the process of forming and strengthening a front).
Bergeron recognized that fronts were found in regions where the streamlines in the
horizontal flow of air form a hyperbolic deformation field. What had been considered an
uninteresting singularity in the field of flow—the neutral point or col—held the key for
understanding where and when fronts form. Bergeron showed that such flows tended to
occur between the semi-permanent highs and lows, explaining the climatological
locations of fronts around the world.
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After completing his doctorate, Bergeron traveled to Malta and the Soviet Union to
lecture on the Bergen School methods. Part II of his doctoral thesis on fronts and their
perturbations was published in Russian in 1934. But his overly critical demands impeded
publishing his own research and completing several books. Still, he produced several
texts that played critical roles in the diffusion of the Bergen meteorology. Although
many of Bergeron’s papers remain unpublished, his popular lecture notes served as
foundations of major textbooks on synoptic meteorology written by his colleagues in
Russian, English, and German.
In 1935 Bergeron failed in his bid to be appointed professor of meteorology at Uppsala
University. Bergeron and his many supporters from abroad could not overcome local bias
against synoptic meteorology, which was considered inferior to laboratory-based
research, as well as a long-standing resentment against Bjerknes and the ‘Norwegian’
achievements with which he was intimately associated. Bergeron returned to Stockholm
in 1936, initially as a meteorologist, but eventually scientific chief of SMHI. He began
giving lectures and exercises based on the Bergen methods. These proved popular,
although at times his perfectionist goals for precision in map analysis and in the wording
of predictions created tension. Legend has it that he insisted on the exclusive use of a
particular brand of colored pencils if accurate weather maps were to be drawn. Within
two years, however, the quality of Swedish forecasting had significantly improved.
Often, meteorologists came from abroad to Bergeron for training. During this time,
Bergeron also served on the Commission of Synoptic Meteorology of the World
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Meteorological Organization, being influential in the development of the international
terminology and classification of clouds and precipitation.
Uppsala University
By the time World War II ended, Sweden had a great shortage of meteorologists,
especially for its rapidly growing aviation interests. Practical weather forecasting classes
were not offered at the universities, and an official report indicated the need for a
professorship in this subject. Although efforts were underway to create such a
professorship for him in Stockholm, in 1947 Bergeron became professor and head of the
Department of Synoptic Meteorology at Uppsala University. Despite Bergeron’s
reputation, finding students was difficult, especially after Carl-Gustaf Rossby established
an active department at Stockholm. Nevertheless, he persisted in lecturing and writing on
the principles of meteorology.
Slowly, he returned to the topic of cloud physics. Bergeron posited that ice crystals could
fall from high-altitude clouds into liquid-water clouds below. Such a seeder–feeder
process could enhance precipitation at the ground. He also wrote about the feasibility of
artificially stimulating the production of precipitation from a synoptic and cloud-physics
perspective.
In 1953, Bergeron started Project Pluvius, a research program designed to better
understand precipitation by establishing high-resolution surface rainfall networks.
Among its rich research results, Project Pluvius showed that modest orography of only
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40–70 m could produce orographic precipitation enhancement. Bergeron retired in 1961,
but continued to work on Project Pluvius. He spent more time traveling and lecturing
worldwide, including several trips to the United States. He died in 1977 of pancreatic
cancer, the last of the original Bergen School meteorologists to pass away.
David M. Schultz and Robert Marc Friedman
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WORKS BY BERGERON
A complete bibliography for Bergeron can be found in Liljequist (1981).
Bergeron, Tor. “Wellen und Wirbel an einer quasistationären Grenzfläche über Europa.
Analyse der Wetterepoche 9.–14. Oktober 1923. [Waves and Vortices at a
Quasistationary Frontal Surface over Europe.] In Veröffentlichungen des
Geophysikalischen Instituts der Universität Leipzig, Series 2, Vol. 3 (1924): 62–
172.
Bergeron, Tor. “Über die dreidimensional verknüpfende Wetteranalyse, I. Tiel.” [ThreeDimensionally Combining Synoptic Analysis, Part I.] Geofysiske Publikasjoner,
5 (1928): 1–111.
Bergeron, Tor. “Methods in Scientific Weather Analysis and Forecasting: An Outline in
the History of Ideas and Hints at a Program.” In The Atmosphere and Sea in
Motion: Scientific Contributions to the Rossby Memorial Volume, edited by B.
Bolin. New York: Rockefeller Institute Press, 1959.
Bergeron, Tor. “Some Autobiographic Notes in Connection with the Ice Nucleus Theory
of Precipitation Release.” Bulletin of the American Meteorological Society 59
(April 1978): 390–392.
WORKS ABOUT BERGERON
Blanchard, Duncan C. “Tor Bergeron and his ‘Autobiographic Notes’.” Bulletin of the
American Meteorological Society 59 (April 1978): 389–390.
Eliassen, Arnt. “Tor Bergeron 1891–1977.” Bulletin of the American Meteorological
Society 59 (April 1978): 387–389.
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Friedman, Robert Marc. Appropriating the Weather: Vilhelm Bjerknes and the
Construction of a Modern Meteorology. Ithaca: Cornell University Press, 1989.
Liljequist, Gosta H. “Tor Bergeron: A Biography.” Pure and Applied Geophysics 119
(1981): 409–442.
Liljequist, Gosta H., ed. Weather and Weather Maps: A Volume Dedicated to the
Memory of Tor Bergeron (15.8.1891–13.6.1977). Basel: Birkhäuser, 1981.
Schwerdtfeger, Werner. “Comments on Tor Bergeron’s Contributions to Synoptic
Meteorology.” Pure and Applied Geophysics 119 (1981): 501–509.
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Figure 1. Schematic of the extratropical cyclone model proposed by the Bergen School
of Meteorology. (From Bjerknes, Jacob, and Halvor Solberg. “Meteorological Conditions
for the Formation of Rain.” Geofysiske Publikasjoner 2 (3, 1921): 3–60.)
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Figure 2. Schematic life cycle of the extratropical cyclone model proposed by the Bergen
School of Meteorology. Dashed lines represent surface fronts; arrows represent
streamlines of the flow. (From Bjerknes, Jacob, and Halvor Solberg. “Life Cycle of
Cyclones and the Polar Front Theory of Atmospheric Circulation.” Geofysiske
Publikasjoner 3 (1, 1922): 3–18.)
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Figure 3. Sketch of the conditions (top) not favoring and (bottom) favoring the Bergeron–
Findeisen process. Dotted areas represent clouds composed of liquid water droplets.
(From Bergeron 1978).
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